Microwave ovens use humidity-sensitive resistors for temperature control during the heating process. It is well known, for example, that foods can be cooked in a microwave oven by controlling the cooking temperature in response to the humidity of the vapor produced as the food is heated.
However, the temperature and the relative humidity in these ovens can vary considerably depending on the ambient conditions. In addition, many conventional humidity-sensitive resistors are overly sensitive to humidity fluctuations so that temperature control in appliances using a conventional humidity-sensitive resistor is less than accurate.
Humidity can be measured by a sensor formed of a material that expands as moisture from a vapor is adsorbed. The output of such sensors is mechanical, however, and can not readily interface with an electronic signal processing device.
Another technique for sensing humidity is with a hygrometer that determines the dew point through use of a probe formed of a material, such as lithium chloride, that functions as a self-regulated heater. The relative humidity is then determined from the difference in the temperatures of the probe and the ambient atmosphere. However, this method requires rather sophisticated electronic equipment that is large and expensive.
A more preferred method of sensing humidity involves use of a porous sintered ceramic metal oxide substrate in the form of a wafer or chip that responds to humidity changes based on variations in electrical resistance (resistivity).
In particular, the adsorption and desorption of water molecules change the surface electrical conductivity of substrates formed of metal oxides. Relative to other types of humidity sensors, sintered ceramic substrates are stable in terms of physical, chemical and thermal properties.
Conventional ceramic humidity-sensitive substrates, such as MgCr.sub.2 O.sub.4.TiO.sub.2 and Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2, produce a linear semilogarithmic relationship between changes in electrical resistance and changes in relative humidity. However, the base resistance of such substrates is very high. Specifically, at lower humidity values (less than about 30 percent relative humidity), the electrical resistance becomes too high for measurement by conventional electronic circuits.
In addition to the resistance limitation, with prolonged exposure to water-containing vapors, these materials tend to form irreversible chemical complexes by hydration of the metal oxide substrate. Therefore, the sensitivity of the device is substantially reduced with time causing what is commonly referred to as an "aging effect". An external heater is often needed to regenerate the sensitivity of the device before each use. See, for example, U.S. Pat. No. 4,080,564 to Nitta et al.
Thus, a need exists for humidity-sensitive materials that respond in a predictable manner to changes in ambient humidity conditions and that operate over a wide range of relative humidity (from 0 to 100 percent). The materials should be relatively free from aging effects. The materials must also accurately measure relatively low electrical resistance values, and thus monitor humidity at low humidity ranges.